superbubble

N70 (also known as Henize 70 and DEM301): a superbubble
located in the Large Magellanic Cloud and measuring about 300 light-years
across.

A superbubble is a large cavity in the interstellar medium created
by the explosion of several supernovae in the
same region of space, typically belonging to the same OB
association. Because the lifetimes of massive O and B stars is measured in only tens of millions
of years, after one supernova has swept clear a bubble around itself, there
isn't enough time for the interstellar medium to back-fill the cavity before
other stars explode in the same region. Each subsequent supernova will rejuvenate
the cavity left by the previous ones, causing the formation of a superbubble
with a diameter of 150 to 300 light-years or more.

The superbubble interior may be quite irregular, containing high velocity
filaments moving chaotically, as seen in the Vela-Puppis region. Other prominent superbubbles in the Milky Way are those surrounding
the Cygnus OB1 association, the Aquila supershell, and the Monogem
Ring. Several superbubbles, surround OB associations in the Large
Magellanic Cloud; these include the N70 superbubble (shown here) and
the N44 superbubble. In fact, most supernova remnants (SNRs) are likely
to be found in superbubbles, but it may be difficult to identify the old
ones individually because they have merged with other old SNRs. We can only
be sure to see the young ones, which are still interacting with circumstellar
gas expelled by their progenitors.